1. Field of the Invention
This invention relates to a photoelectric position indicator comprising a scale phase grating, and a scanning unit comprising at least one scanning phase grating, which is movable relative to the scale phase grating, at least one light source, which is movable in unison with the scanning phase grating or gratings and a collimator, and photodetectors. Interference patterns having groups having a predetermined order of diffraction are generated by the light which has been diffracted several times during a movement of the scanning unit along a measuring direction, and produce periodic phase-displaced actual-value signals on the photodetectors in dependence on the orders of diffraction of said groups during a movement of the scanning phase grating or gratings and the scale phase grating relative to each other.
2. Description of the Prior Art
Such a position indicator is known from EP-A-0 509 979. In the operation of such position indicators, the principle of the so-called three-grating triple-diffraction pick-up is adopted, the Fundamentals of which have been explained in detail in the dissertation by J. Wilhelm "Dreigitterschrittgeber-photoelektrische Aufnehmer zur Messung von Lageanderungen" (Hanover University of Technology) published in 1978. If reflecting phase gratings are used as a scale, the light will be diffracted twice by the scanning phase grating. If a transparent scale is employed, another scanning phase grating may be provided on the second side of the scale. Finally, the light which has been diffracted by the first scanning phase grating and the transparent scale may be directed by a deflecting prism provided on the rear side of the scale along a portion of the length of the scale and then through the scale and a further scanning grating to the detectors; in that case the light is diffracted twice by the scale.
In accordance with EP-A-0 509 979 signal pairs which are displaced 90.degree. in phase and comprise each two phase-opposed signals are generated and for that purpose the scale member is provided with two juxtaposed scale tracks, which are offset by 1/8 of the grating increment. The scanning grating is common to both scale tracks and two photodetectors are associated with each scale track and serve to receive the interference patterns of the light beams which have been diffracted in that direction so that the signals of the pairs of signals can be combined in antiparallel connections for the generation of analog actual-value signals which lead or lag relative to each other in dependence on the direction of movement. In that case it is possible to use evaluating circuits which are similar to those used in conventional incremental measuring systems.
In all three-grating triple-diffraction pick-ups of the previous type, phase errors may occur, particularly owing to pitch errors of the several gratings and/or tolerances regarding the mounting of the scanning gratings relative to the scale grating or gratings. For this reason, time-consuming adjusting work is required to ensure a generation of useful signals as a result of the measurement.
In order to control the phase error and nevertheless to generate signal pairs which can be evaluated and have a suitable phase offset, a measuring system disclosed in Patent Abstracts of Japan, Vol. 7, No. 10 (P-168) (1155) of Jan. 14, 1983, and JP-A-57 169 613 comprises a phase grating as a scale and two measuring systems, which are coupled to each other and consist each of a light source, a collimator, scanning phase gratings and photodetectors. At least the light source of the one measuring system or one entire measuring system can be adjusted relative to the other measuring system in the direction of the scanning movement. That design is highly expensive and has the basic disadvantage that the two measuring systems necessarily process signals from scale points which are longitudinally spaced a large distance apart and may possibly be different owing to pitch errors.